Things to Know (#’s)


# Species Concepts (3)

Snow day guide: Write out the criteria for each concept, then write out one flaw


# Barriers to Mating

Snow day guide: This is an extension of the biological species concept. Focus on being able to recognize/write out scenarios that prevent mating from occurring (pre-zygotic = habitat, temporal, behavioral, and mechanical isolation)


#Allopatry vs Sympatry

Snow day guide: These are two ‘mechanisms’ by which species actually form (different from above). Be able to recognize/write out scenarios that would form new species for each

Macroevolution





  • Microevolution happens at the population level (natural selection, genetic drift, gene flow)


  • Macroevolution: Speciation events form evolutionary independent populations


  • New species are the bridge between evolution within a population and larger patterns in evolution, such as new taxonomic groups

How/Why species form: The limits of human cognition

What actually is a “Species”?







  • Textbooks: A group of populations whose members have the potential to interbreed in nature and produce viable offspring

What actually is a “Species”?







  • Textbooks: A group of populations whose members have the potential to interbreed in nature and produce viable offspring

Biological Species Concept: Unity of reproduction


  • Reproductive isolation between populations results in a lack of gene flow
    • what are the consequences of no gene flow?


  • Individuals do not interbred with other populations or are unable to produce viable offspring after mating
    • via pre- and post-zygotic barriers
      • prevent mating (pre) or prevent viable offspring (post)


  • BSC: Members of a biological speci es are united by the potential ability to be reproductively compatible

Mechanisms of reproductive isolation in Meadowlarks


Issues with the biological species concept



  • How do we effectively evaluate isolation?
    • Is this even possible do to in fossils?


  • Geographic proximity: how do we know if they cant make babies?


  • Asexual reproduction is unaccounted for…
    • Belloid rotifers haven’t reproduced sexually for > 80 million years
    • An estimated 2000 species of asexual rotifers are known

Morphological Species Concept: Unity of body plans

MSC: defines a species based on groups of physical traits that are unique to each species
MSC: no issues with fossils or reproductive type

Issues with the morphological species concept




  • Applies to all organisms but…. what kinds of traits & how many?
    • Subjective to the scientist!


  • Species can change their form: Polymorphisms


  • Cryptic species that do not differ in morphology
    • lots of mimics in nature!

Ecological species concept: Unity of Niche




  • ESC: defines a species in terms of its ecological niche
    • Niche = sum of how members of the species interact with the nonliving and living parts of their environment
    • living map of the habitat + other species interactions any species requires


  • Key idea: members of the same species occupy the same niche and are shaped by the same selection pressures
    • different species would occupy different niches
    • any new species should have a ‘new niche’ compared to its ancestor

Issues with the ecological species concept


  • Niches can overlap or shift = niche boundaries can be fuzzy
    • many forest birds both eat the same insects in the same trees
    • a fish species may eat insects in streams but switch to zooplankton in lakes


  • Humans define a species’ niche based on ‘X’ factors they can measure
    • more/less factors can redefine the niche


  • One species can occupy multiple niches across life stages or environments—making boundaries fuzzy
    • tadpole & adult frog have different niches

How new species form: Allopatry

Allopatric speciation: the formation of new species due to geographic (physical) isolation that stops gene flow between populations

Allopatric speciation in Hawaiian fruit flies

Hawaiian islands form through time. Fruit flies from one population migrate to a new island (physical separation) and have formed many new species.

How new species form: Sympatry

Sympatric speciation: the process by which new species evolve from a single ancestral population while inhabiting the same geographic location

Sympatry in Hawthorn and apple flies

How much genetic change does it take to form a new species?


  • Three spine stickleback fish rapidly evolve
    • moved from oceans to freshwater when glaciers retreated after last ice age


  • Freshwater habitats brought new selection pressures
    • variety of habitats
    • new resources
    • change in competition


  • Molecular biology examined each speciation
    • ocean → fresh: one gene with bony armor plate
    • fresh habitats → many genes based on selection pressures

Study aid: The geography of speciation

The pre- and postzygotic barriers can form for either type of speciation